Disk drives are an important data storage technology. Read-write heads are one of the crucial components of a disk drive, directly communicating with a disk surface containing the data storage medium.
FIG. 1A illustrates a typical prior art high capacity disk drive 10 including actuator arm 30 with voice coil 32, actuator axis 40, suspension or head arms 50–58 with slider/head unit 60 placed among the disks.
FIG. 1B illustrates a typical prior art high capacity disk drive 10 with actuator 20 including actuator arm 30 with voice coil 32, actuator axis 40, head arms 50–56 and slider/head units 60–66 with the disks removed.
Since the 1980's, high capacity disk drives 10 have used voice coil actuators 20–66 to position their read-write heads over specific tracks. The heads are mounted on head sliders 60–66, which float a small distance off the disk drive surface when in operation. Often there is one head per head slider for a given disk drive surface. There are usually multiple heads in a single disk drive, but for economic reasons, usually only one voice coil actuator.
Voice coil actuators are further composed of a fixed magnet actuator 20 interacting with a time varying electromagnetic field induced by voice coil 32 to provide a lever action via actuator axis 40. The lever action acts to move head arms 50–56 positioning head slider units 60–66 over specific tracks with speed and accuracy. Actuator arms 30 are often considered to include voice coil 32, actuator axis 40, head arms 50–56 and head sliders 60–66. Note that actuator arms 30 may have as few as a single head arm 50. Note also that a single head arm 52 may connect with two head sliders 62 and 64. Introduced in the 1990's, merged heads brought significant increases in areal density. A merged type head reads the disk surface using a spin valve. The spin valve contains a conductive thin film, whose resistance changes in the presence of a magnetic field. By separating the functions of writing and reading, each function can be optimized further than would be possible for the older read-write heads. For all the improvement that merged heads bring, there remain problems. However, before discussing these problems, consider first how and what controls these devices in contemporary disk drives.
Merged type heads possess different components for reading and writing, because the magneto-resistive effect only occurs during reading. A merged type head typically includes a thin film head and a spin valve sensor. The primary use of the thin film head is in the write process. The spin valve sensor is used for reading.
Merged Magneto-Resistive (MR) heads have several advantages over earlier approaches, which used a single component, for both read and write. Earlier read-write heads were a study in tradeoffs. The single component, often a ferrite core, can increase read sensitivity with additional windings around the core. However, these added windings make the ferrite core write less efficiently.
Introduced in the 1990's, merged heads brought significant increases in areal density. A merged type head reads the disk surface using a spin valve, containing a conductive thin film, whose resistance changes in the presence of a magnetic field. By separating the functions of writing and reading, each function can be optimized further than would be possible for the older read-write heads. For all the improvement that merged heads bring, there remain problems. However, before discussing these problems, consider first how and what controls these devices in contemporary disk drives.
FIG. 2A illustrates a suspended head slider 60 containing the MR read-write head 200 of the prior art, which is part of the actuator assembly as shown in FIGS. 1A–1B.
FIG. 2B illustrates a simplified schematic of a disk drive controller 1000. Disk drive controller 1000 controls an analog read-write interface 220 communicating resistivity found in the spin valve within MR read-write head 200. Disk drive controller 1000 concurrently controls servo-controller 240 driving voice coil 32 of the voice coil actuator to position merged read-write 200 to access a rotating magnetic disk surface 12 of the prior art.
Analog read-write interface 220 frequently includes a channel interface 222 communicating with pre-amplifier 224. Channel interface 222 receives commands setting at least the read—bias, write—bias, and thermal asperity detection threshold(s), denoted as TA—threshold in FIG. 2B.
Various disk drive analog read-write interfaces 220 may employ either a read current bias or a read voltage bias. By way of example, the resistance of the read head is determined by measuring the voltage drop (V—rd) across the read differential signal pair (r+and r−) based upon the read bias current setting read—bias, using Ohm's Law.
Control of the disk drive requires rapid and dynamic feedback and control of the voice coil 32, which is usually done by a servo controller 240, responding to commands from the embedded disk controller 1000. In some hard disk drives, the servo controller is physically part of the embedded disk controller. It has been illustrated as separate strictly to facilitate the subsequent discussion of the central features of the invention, and is not meant to imply a limitation of scope upon the claims.
The servo controller is given directions on where to position the read-write head 200 to access the rotating disk surface 12 of FIG. 1A. Merged read-write head 200 accesses data organized into tracks, each track containing several sectors. Because the disk surface is rotating, the positioning of the read-write head requires both an angular and radial positioning of the read-write head to access a sector of a track of the disk drive. Typically, servo controller 240 provides a head speed feedback driven control of voice coil 32 to position the read-write head 200 with respect to a rotating disk surface 12.
Historically, there have been two distinct traditions regarding the physical arrangement of data on a data storage surface, one dominated by audio storage and the other dominated by digital data storage. All magnetic disk drives, since the start of computing, have used a fixed radius for each track. Before discussing the invention, consider the history of the prior art.
FIG. 3A illustrates a spiral track as found in prior art technologies including phonograph records and compact disks.
Audio data storage employs a spiral track arrangement for storing data with a continuous spiral of data. The earliest audio storage technology using this approach was Edison's gramophone using cylinders, which went into production before the twentieth century. Early in the twentieth century, Edison and others put the disc phonograph into production. The disc phonograph also used a spiral pattern for audio recording by mechanically accessing that pattern on a flat, rotating disc surface. A subsequent application of this approach is found in contemporary compact disks, which use an is optical method to access information again stored in a spiral arrangement on a rotating, flat surface.
FIG. 3B illustrates a circular track arrangement as found in random access, digital storage devices include all magnetically recorded disk drives.
Digital data storage begins with the first electronic computers by the early 1950's. Earliest versions of digital data storage used circular tracks magnetically recorded on drums, which were followed by the use of concentric, circular tracks magnetically recorded on rotating disk surfaces.
There were excellent reasons as to why these two distinct approaches evolved over the last several decades.
Audio data is by its nature sequential and often of indefinite length. By way of example, the movements of a symphony are not all of the same length, and each movement is specifically performed sequentially in time. The requirements for audio data storage are driven by these two facts, whether in Edison's laboratory, or in the audio compact disk of today. The spiral track recording arrangement is a great way to satisfy these requirements.
Data storage is driven by the needs of computers which have, since early in their evolution, stored information in databases and file management systems. These organization tools are predicated upon the ability to randomly access anywhere within a collection of data units, often known as sectors or records. To facilitate this approach, early computer manufacturers devised a distinctive approach, involving circular tracks, each containing a fixed number of sectors, which could be independently written. These sectors were then integrated into files and database objects, which were then further organized into directories, folders, and so on.
The requirement of computer-oriented digital data storage is to move freely throughout the stored data, modifying any record or sector at will, without excessive overhead to the whole. The circular track approach has proven to be reliable and has facilitated a revolution in computer storage technology.
Methods were invented for using compact disks to act like a disk drive by the late 1990's. These methods permit data to be written on compact disks, and then read in a fashion compatible with disk management systems such as found in contemporary operating systems. These methods employed the audio-based standard of using spiral tracks, recording a variable number of packets or sectors on each of these tracks. However, these methods were not designed to provide a random writing of a sector within a track, but rather to provide archival and offline storage capabilities.
It turns out that these traditional audio storage devices are incapable of being used as randomly written digital storage devices comparable to magnetic hard disk drives. These devices just cannot be easily written with random sequences of sectors.
While the magnetic disk drives have performed their task quite well, there is a persistent inefficiency associated with them. The read-write head must traverse a radial distance to go from one track to its successor. While traversing the gap between these tracks, no data can be accessed. The access bandwidth is essentially halted until the read-write head is again positioned over the successor track. The time to traverse the gap between successive tracks adds about 20% to the time it takes to access all the data on the track.
What is needed is a magnetic disk drive supporting the requirements of computing without halting access of successive tracks.